Science Musings Blog

Thursday, April 06, 2006

March of the Stones

A few weeks of wet weather, freezing nights, and daytime thaws and the stones are rising up out of my path. They burgeon from the soil like lithic cabbages, shouldering aside the frozen earth.

It's called frost heaving. Wet soil beneath a buried stone freezes and expands, lifting the stone and creating a cavity underneath. Pebbles or grit sift into the cavity. When the ground thaws the stone is prevented from settling into its old place. It has been lifted, ever so slightly. Another freeze, another thaw: the cycle is repeated. Millimeter by millimeter the stone makes its way to the surface, emerging into the light of day.

Once on the surface, stones make their way downhill. Freezing water lifts a stone in a direction perpendicular to the slope of the hill. Then, as the soil thaws, gravity pulls the stone straight down. Up forward, straight down. Up forward, straight down. Thus do stones descend to the bottom of sloping meadows, taking their sweet geologic time, creeping on icy fingers.

Robert Thorson, a geologist at the University of Connecticut, has a theory about the stone walls of New England: Many of them are waste dumps for stones heaved up out of the ground by the deep frosts that occurred after the region's trees were cut down in the 18th and early 19th century. Prior to the great deforestation, soil was insulated from deep freezing by trees, leaves, and snow. With the clearing of the trees, stones were waked from a subterranean slumber that had been undisturbed since the end of the last ice age.

According to Thorson, many of New England's storied stone walls were not built to fence in livestock or mark boundaries, but to dispose of rocks that frost heaving popped out of the ground.